The present invention relates to lubricating oil compositions, and more particularly to crankcase lubricant compositions which contain an effective fuel economy improving additive.
There is an increased requirement for lubricant compositions which are capable of improving the fuel economy of the internal combustion engines in which they are used. An improvement in fuel economy, i.e., a reduction in fuel consumption, generally requires a lowering of frictional losses under a range of lubrication regimes. These regimes are known to those skilled in the art and may be defined in terms of the extent to which lubricant film thicknesses formed in the various points of contact within an engine exceed or fail to exceed the combined roughness of the contact surfaces.
The film thickness depends, in part, on contact geometry, load, elastic properties of metals, lubricant viscosity and the speed with which a lubricant is entrained into the points of contact. Generally speaking, film thickness increases as the viscosity of the lubricant increases and as the speed of sliding and/or rolling motion between the points of contact increases. The increase of the film thickness is not linear, however, and well established equations for predicting film thickness under elastohydrodynamic conditions indicate that the film thickness increases at approximately the same rate as the viscosity to the 0.7 power increases, i.e., viscosity.sup.0.7, and at approximately the same rate as the speed of sliding and/or rolling contact to the 0.7 power increases, i.e. speed.sup.0.7. Dowson D. and Higginson G., "Elastohydrodynamic Lubrication", Pergamon Press, Oxford, England, 1977; and Hamrock, B. and Dowson, D., "Ball Bearing Lubrication: the elastohydrodunamics of eliptical contacts", J. Wilet, N.Y., 1981. In accordance with these well established equations, ideal behavior would be characterized by a linear increase in elastohydrodynamic film thickness when plotted against entrainment speed on a log basis, i.e., a straight line (referred to herein as the or a "theoretical line") having a slope of about 0.7.
The lubrication regimes which need to be considered are (1) the hydrodynamic regime, (2) the mixed regime, and (3) the boundary regime. The hydrodynamic regime occurs when the contact surfaces are separated by a lubricant film which is thick by comparison with the roughness of the contact surfaces. This condition occurs when contact pressures are low and/or when speed and/or lubricant viscosity are high. The frictional losses which occur under hydrodynamic conditions are generally proportional to the viscosity of the lubricant at the points of contact. Thus, for increasingly more viscous lubricants, there will be increasingly thicker lubricant films at the contact points, such that there will be a correspondingly lower probability of metal to metal contact and wear. However, as the viscosity of the lubricant increases, there will also be a corresponding increase in frictional losses due to the energy required to shear the thicker lubricant films. When operating under hydrodynamic conditions, frictional coefficients, also known as traction coefficients, typically are on the order of about 0.07 to about 0.03. The lower values are beneficial for fuel economy.
As speeds fall, as contact pressures rise, or as lubricant viscosity falls, the lubricant film thickness generated for a given contact geometry will decrease to the extent that it approaches the dimensions of the surface roughness encountered by the lubricant. Under these conditions the lubricant is operating in the mixed regime and frictional losses are in part due to metal to metal contact and in part due to lubricant shearing friction. Metal to metal contact results in high friction losses and wear, whereas lubricant shearing friction results in lower friction losses. Typically, friction coefficients due to lubricant shearing are on the order of about 0.03, whereas friction coefficients due to metal to metal contact are on the order of from about 0.08 to about 0.30. Thus, as the lubricant film thickness/surface roughness ratio decreases, the contribution to friction loss due to metal to metal contact becomes dominant and the combined friction coefficient (from metal to metal contact and lubricant shear) rapidly increases, typically from about 0.03 to about 0.05-0.15 over a narrow range of lubricant film thickness. In other words, when operating under the mixed lubricant regime, there is a rapid increase in friction losses with a relatively small decrease in lubricant film thickness. Accordingly, any lubricant formulation which enables operation under fluid lubrication to occur down to lower speeds will be beneficial both as to wear and fuel economy. This is especially true if the friction (traction) losses due to the properties of the lubricant are minimized. The difficulty, however, is to get low friction, high viscosity lubricant films into the contact areas when operating at lower speeds.
When speeds are very low, or when lubricant viscosities are very low and/or when contact pressures are very high, the lubricant film thicknesses generated in the contact areas fall to values very much less than the roughness of the contact surfaces. Under these conditions, referred to as the boundary friction regime, the friction losses depend on the properties of surface films formed by physical and/or chemical processes at the contact surfaces. Depending on the properties of the films so formed, the friction coefficients under boundary conditions for contact surfaces lubricated with oil formulations typically are in the range of from about 0.05 to about 0.15. It is known in the art that what are normally referred to as friction modifiers, e.g., glycerol monooleate, are effective for reducing friction losses under boundary lubrication conditions.
The hydrodynamic lubrication regime, the mixed lubrication regime and the boundary lubrication regime occur simultaneously in internal combustion engines at any given time. Depending on the contact geometry, the speeds of sliding and/or rolling contact, the load and the lubricant oil viscosity and temperature, the friction losses can be described in terms of the contribution from the various lubrication regimes, bearing in mind that the contributions will vary for any given lubricant oil as the operating conditions of the engine vary.
One way to illustrate the effects of the various lubricating regimes is to plot the friction coefficient versus the contact speed (or the lubricant film thickness, which is proportional to the contact speed). Such a plot, referred to as a Stribeck traction curve, is useful for comparing the friction losses expected from use of one lubricant formulation over another. A typical Stribeck traction curve (see FIG. 1) will show that the friction coefficient will decrease rapidly with increasing speed (or lubricant film thickness) at very low speeds, and then will level out, and possibly increase slightly, as speeds (or lubricant film thickness) increase. The integrated area under the Stribeck traction curve is a measure of the total friction loss and can be used to project the relative fuel consumption requirements of various lubricant formulations.
There are a number of prior art disclosures relating to the addition of friction modifiers and other additives to lubricating oil compositions with an eye toward reducing friction losses and engine wear. U.S. Pat. No. 2,493,483 to Francis, for example, relates to lubricants for marine steam engines which form oil in water emulsions. The lubricants include "secondary additives" which function to improve performance under certain severe and adverse conditions. The secondary additives comprise esterified polyhydric alcohols, such as glycerol mono- and dioleate, sorbitan mono-, di and trioleate, and pentaerythritol monooleate.
U.S. Pat. No. 2,783,326 to Bondi relates to lubricants usable under extreme operating conditions, e.g., extreme pressure conditions, high speeds, high temperature gear and bearing protection, etc. The lubricants, which are suitable for transmission applications, contain extreme pressure additives and solubilizing agents for the extreme pressure additives. The solubilizing agents may comprise non-ionic esters such as glycerol monooleate, sorbitan monooleate and pentaerythritol monooleate.
U.S. Pat. No. 3,235,498 to Waldmann discloses the use of an ester additive such as glycerol monooleate or sorbitan monooleate to inhibit the foaming tendency that might otherwise occur in lubricating oil formulations which include one or more detergents.
U.S. Pat. No. 3,933,659 to Lyle relates to transmission fluids which contain a number of additives, including fatty esters of dihydric and other polyhydric alcohols, such as pentaerythritol monooleate.
U.S. Pat. No. 4,175,047 to Schick discloses the addition of from 20-40% of a hydroxy-containing ester to a lubricating oil composition as a fuel consumption reducing agent. The improvement in fuel economy is said to be the result of a reduction of viscous friction (which would be beneficial under hydrodynamic conditions). The esters of this patent are derived from acids having a carbon chain length of from about 5 to about 30 carbon atoms and include, for example, glycerol monooleate and sorbitan monooleate. There is no discussion in this patent as to the viscosity of the usable esters, nor of any possible performance advantage under boundary and/or mixed lubrication conditions.
U.S. Pat. No. 4,304,678, also to Schick, relates to the addition of from about 1 to about 4% of a hydroxy-containing ester to a lubricating oil to improve fuel economy. The improvement is said to be the result of reduced friction under boundary lubrication conditions. There is no discussion in this patent as to the possible effects under hydrodynamic or mixed lubrication conditions. The esters disclosed in this patent include glycerol monooleate and sorbitan monooleate.
U.S. Pat. No. 4,336,149 and U.S. Pat. No. 4,376,056, both to Erdman, relate to the addition of from about 0.25 to 2 wt. % of pentaerythritol monooleate to a crankcase lubricating oil to increase the fuel economy. These patents indicate that gains in fuel economy through the use of additives to reduce friction under mixed regime conditions probably will be small and difficult to assess.
U.S. Pat. No. 4,734,211 to Kennedy relates to lubricating oil compositions for use with railway diesel engines, which typically have silver plated bearings. The lubricant compositions include base oil, a dispersant, at least one overbased detergent, and a polyhydroxy compound such as glycerol monooleate or pentaerythritol trioleate to inhibit silver wear.
U.S. Pat. No. 5,064,546 to Dasai relates to lubricating oils which reduce friction in transmission, wet clutch and shock absorber applications. The lubricating oils contain a specific base oil and a friction modifier such as a fatty acid ester of sorbitan, pentaerythritol, trimethylol propane, or the like.
U.S. Pat. No. 4,683,069 to Brewster relates to lubricating oil compositions which exhibit improved fuel economy and which contain from about 0.05 to 2 wt. % of a glycerol partial ester of a C.sub.16 -C.sub.18 fatty acid.
U.S. Pat. No. 4,105,571, U.S. Pat. No. 4,459,223 and U.S. Pat. No. 4,617,134, all to Shaub, relate to lubricating oil compositions having improved friction reducing and anti-wear properties. The '571 patent discloses a composition comprising a base oil and a predispersion of a glycol ester and/or a zinc dihydrocarbyl dithiophospahte with an ashless dispersant to improve package stability. The '223 patent discloses the use of up to about 2 wt. % of an ester additive, which is derived from dimer carboxylic acids and polyhydric alcohols having at least three hydroxy groups, to reduce boundary friction. The '134 patent discloses the use of less than 2 wt. % of an ester of a polycarboxylic acid with a glycol or glycerol, plus an ashless dispersant and a zinc dihydrocarbyl dithiophosphate to reduce boundary friction.
U.S. Pat. No. 4,167,486 to Rowe relates to lubricating oils containing olefin polymerizable acid esters and dimers and/or trimers thereof as fuel economy improving additives. The esters disclosed in this patent contain at least two double bonds paired in one of the following configurations: --C.dbd.C--C--C.dbd.C-- or --C.dbd.C--C.dbd.C--. The esters disclosed in this patent and are distinguishable from esters of oleic acid, for example, which have only one double bond, i.e., --C.dbd.C--, per alkyl chain length.
U.S. Pat. No. 4,440,660 to Van Rijs describes low viscosity esters for use in lubricating oils. The esters typically would have a viscosity lower than the viscosity of the base oil.
U.S. Pat. No. 4,154,473 to Coupland discloses the use of molybdenum complexes to reduce friction. This patent mentions reduction of friction losses by use of synthetic ester oils, but there are no details given as to the which esters might be used, as to the viscosity of the esters, nor as to the their contemplated treat rates.
In spite of the many advances in lubricant oil formulation technology, there remains a need for lubricant oil compositions that offer improved fuel economy.